Melt chocolate in double boiler and stir till smooth, set aside to cool.
In a large bowl, beat egg yolks lightly with a hand whisk, followed by oil, milk and melted chocolate. Add vanilla extract, stir well.
Combine flour, cocoa powder, baking powder and baking soda together and sift into the egg yolk mixture, mix till combined and smooth.
Whisk egg whites at low speed till foamy, add lemon juice into it. Then gradually add in sugar and beat at medium high speed to stiff peak.
Lastly fold egg whites into the yolk mixture in 3 batches till well combined.
Pour the batter in a 7 inch removable cake pan. Bake in preheated oven at 165 deg C for about 50 minutes at 2nd lower rack in the oven.
Invert cake pan immediately and let to cool down after baked.
Chocolate mousse - Melt chocolate indouble boiler and set aside. Boil 50ml whipping cream at medium heat and mix into melted chocolate, stir well with a hand whisk followed by vanilla extract and kahlua liqueur.
Whisk 100ml fresh topping cream and 100ml whipping cream together to peak form (not too stiff). Fold into chocolate mixture with a rubber spatula.
Assemble cake - Cut away the top of the chocolate cake to slightly flat. Then cut cake into 3 slices.
Place one slice of cake on a cake board. Spread chocolate mousse on top and cover with the 2nd piece of cake over it. Spread chocolate mousse again followed by the 3rd piece of cake and chocolate mousse on top. Flatten the top and the sides of the cake with a spatula or scraper. Then chill the cake in refrigerator for at least 3 hours or overnight.
Ganache cream - Add chocolate and butter into a bowl. Boil whipping cream at medium and pour into chocolate. Stir well with a hand whisk to smooth. Lastly add vanilla extract and kahlua liqueur and stir well again. Leave ganache cream to cool..
Take the mousse cake from fridge and pour a layer of ganache cream on top and spread it with a spatula. Then place the mousse cake back into the fridge for 15 minutes and spread another layer of ganache cream on top (as shown in pic). Decorate the chocolate mousse cake as desired.
Notes
if you prefer to have a sweeter cream frosting, use all 200ml fresh topping cream instead of 100ml of whipping creamchiffon cake recipe is the same here